The lithium-sulfur batteries provide for a promising energy storage system due to their superior specific capacity (1675 mAh per gram of sulfur). However, such batteries pose several technological challenges. In particular, rapid capacity fade and low coulombic efficiency (CE) plague the batteries. These challenges are believed to be associated with the loss of the sulfur active material during repeated charging and discharging, and through dissolution of the lithium polysulfides from the sulfur cathode into the electrolyte. Such polysulfides may then undergo side reactions with the electrolyte solvents and the lithium anode.
Lithium-sulfur cells operate as lithium ions migrate from the lithium metal anode surface during discharge, and to the anode during charging. This in contrast to conventional lithium-ion cells where the ions are intercalated in the anode and cathode, resulting in much higher lithium storage density. With a sulfur cathode (a composite with a conductive material to account for sulfur, itself, being an insulator), during discharge the sulfur is reduced to a variety of polysulfides and eventually to lithium sulfide according to:S8→Li2S8→Li2S4→Li2S2→Li2SMajor technical roadblocks for lithium-sulfur batteries have been that polysulfides that dissolve into the electrolyte and diffuse to the anode where they cause severe shuttling effects, the lithium sulfides are insoluble and plate onto the cathode blocking active material underneath, and severe self-discharge.
One previous approach to reducing these effects was to introduce porous carbon materials into the cathode to trap the lithium polysulfides within the cathode during cycling by the strong adsorption property of carbon. See e.g. Ji et al. J. Mater. Chem. 20 (2010) 9821; Jeong et al. J. Power Sources 235 (2013) 220; and Zu et al. Phys. Chem. Chem. Phys. 15 (2013) 2291. Another approach was to form a protective layer on the lithium anode surface to mitigate the redox reaction of the dissolved polysulfides and lithium metal. See e.g. Chung et al. J. Electroanal. Chem. 566 (2004) 263; Lee et al. J. Power Sources 119-121(2003) 964; and Mikhaylik et al. ESC Trans. 25 (2010) 23. Yet another approach was the development of new solid state electrolyte, (See Nagao et al Energy Technol. 1 (2013) 186-192, and Hassoun et al., B. Scrosati, Adv. Mater. 22 (2010) 5198), electrolytes consisting of ionic liquid, (See e.g. Yuan et al. Electrochem. Commun. 8 (2006) 610.; Shin et al. J. Power Sources 177 (2008) 537; Dokko et al. Electrochem. Soc. 160 (2013) A1304), tetra(ethylene glycol) dimethyl ether (Ryu et al. J. Power Sources 163 (2006) 201, and Chang et al. J. Power Sources 112 (2002) 452), as organic solvents for the electrolyte, lithium salt electrolytes, (See e.g. Aihara et al. J. Electrochem. Soc. 151 (2004) A119), and functional electrolyte additives (See e.g. Lin et al. Adv. Funct. Mater. 23 (2013) 1064) to prevent the dissolution of the polysulfides into the organic electrolyte and thereby avoid the redox shuttling effect. These approaches can improve the Li—S performance to some extent but still cannot solve the above-mentioned problems.